Cellulose ester has been utilized as a supporter (or a base material) of a photosensitive photographic material, a polarizer protective film of liquid crystal displays (LCDs), a retarder film, a color filter, and others due to the excellent optical properties. In such an optical film, there has been also investigated on an affect of acyl group to a substitution position in a glucose unit constituting cellulose ester.
Among such cellulose ester, cellulose acetate, particularly cellulose triacetate, has been widely used. Cellulose triacetate is particularly poor in solubility, and a practically usable solvent for dissolving cellulose triacetate has been substantially limited to methylene chloride. Further, solubility changes depending on a degree of substitution of cellulose acetate. Therefore, even in the case of using methylene chloride as the solvent, there has been used a cellulose acetate having a degree of substitution (total of degrees of substitution) of 2.7 to 2.9 and acetic acid content of 59 to 61%. That is, the cellulose triacetate in which almost all hydroxyl groups are acylated, i.e., the cellulose triacetate which is close to a tri-substituted product having a degree of substitution of larger than 2.9, could not be practically used in the view of solubility.
For example, Japanese Patent Application Laid-Open 5851/1999 (JP-11-5851A, Patent Document 1) discloses films which comprise a cellulose acetate having a total of degrees of substitution of acetyl groups at 2-, 3- and 6-positions of not less than 2.67 (e.g., not less than 2.77) and having a total of degrees of substitution of acetyl groups at 2- and 3-positions of not more than 1.97. This document describes that use of the cellulose acetate ensures preparation of a stable solution by a cooling dissolution method even in the case of using a non-halogen solvent and enables production of a film having a low retardation value in the thickness direction and being suitable for a polarizer protective film by a flow-casting.
Japanese Patent Application Laid-Open No. 212338/2002 (JP-2002-212338A, Patent Document 2) discloses a cellulose acylate in which substitution degrees of acyl groups at 2- and 3-positions are in the range of 1.70 to 1.90 in total and a substitution degree of acyl group at 6-position is not less than 0.88, and a film thereof. Japanese Patent Application Laid-Open No. 265501/2002 (JP-2002-265501A, Patent Document 3) discloses a cellulose acylate in which substitution degrees of acyl groups at 2-, 3- and 6-positions are not less than 2.67 in total, substitution degrees of acyl groups at 2- and 3-positions are not more than 1.97 in total, and the following formula: −0.1≦[(a degree of substitution of acyl group at 3-position) −(a degree of substitution of acyl group at 2-position)]≦0.3 is satisfied. This document also discloses a cellulose acylate in which substitution degrees of acyl groups at 2- and 3-positions are 1.70 to 1.90 in total, a substitution degree of acyl group at 6-position is not less than 0.88, and the following formula: −0.1≦[(a degree of substitution of acyl group at 3-position)−(a degree of substitution of acyl group at 2-position)]≦0.3 is satisfied. These documents mention that cellulose acylate solution which is excellent in stability with time and has a low viscosity in a practical concentration region can be obtained and that a film having a high surface smoothness can be obtained by flow-casting the solution.
Japanese Patent Application Laid-Open No. 309009/2002 (JP-2002-309009A, Patent Document 4) discloses a cellulose acylate film which comprises a mixed polymer comprising a cellulose acylate in which substitution degrees of acyl groups at 2- and 3-positions are 1.7 to 1.95 in total and a substitution degree of acyl group at 6-position is not less than 0.88, and a cellulose acylate in which substitution degrees of acyl groups at 2- and 3-positions are 1.7 to 1.95 in total and a substitution degree of acyl group at 6-position is less than 0.88. Japanese Patent Application Laid-Open No. 105129/2003 (JP-2003-105129A, Patent Document 5) discloses a cellulose acylate film which comprises a cellulose acylate in which substitution degrees of acyl groups are 2.7 to 2.9 in total, substitution degrees of acyl groups having 3 to 22 carbon atoms are 0.4 to 2.5 and a substitution degree of acyl group at 6-position is less than 0.9, and a cellulose acylate in which substitution degrees of acyl groups are 2.75 to 2.9 in total, substitution degrees of acyl groups having 3 to 22 carbon atoms are 0.0 to 0.4 and a substitution degree of acyl group at 6-position is not less than 0.9. Use of these cellulose acylates ensures a dope which is high in solubility and low in solution viscosity.
Japanese Patent Application Laid-Open No. 338601/2002 (JP-2002-338601A, Patent Document 6) discloses a process for preparation of a cellulose acetate which comprises the steps of: reacting a cellulose in a solvent with acetic acid or acetic anhydride in the presence of a catalyst to synthesize a cellulose acetate; and ripening the synthesized cellulose acetate in the presence of a catalyst, an acetyl donor, and water or an alcohol, and under a condition that the amount of water and the alcohol is in the range of 0.1 to 10 mol % based on the amount of the acetyl donor (claim 1); a cellulose ester in which the degrees of acetyl substitution at 2-, 3- and 6-positions satisfy the following formulae: 2DS+3DS>1.80, 3DS<2DS, and 6DS>0.80, in which 2DS is the degree of acetyl substitution at 2-position, 3DS is the degree of acetyl substitution at 3-position, and 6DS is the degree acetyl substitution at 6-position (claim 4). For example, in Example 15, a cellulose acetate satisfying 2DS=0.973, 3DS=0.967, and 6DS=0.967 is obtained. This document describes that a total of degrees of substitution of acetyl groups at 2- and 3-positions and a degree of substitution of acetyl group at 6-position can be adjusted, and in addition, a degree of substitution of acetyl group at 2-position and a degree of substitution of acetyl group at 3-position can be adjusted; a solubility in an organic solvent and a viscosity of a dope can be controlled; and a film excellent in optical properties can be obtained.
Thus, various attempts have been made so that to adjust a degree of acyl group as a substituent may improve optical properties of a cellulose ester film (particularly a cellulose acetate film), and further improvement in the optical properties (particularly optical isotropy) has been required. Moreover, attempts for improving solubility or stability of the solution have been made in the case of using a general non-halogen solvent without using a halogen-containing solvent such as methylene chloride.
On the other hand, recently, in a liquid crystal drive system of liquid crystal displays, to use VA (Vertically Aligned) mode or OCB (Optically Compensated Bend) mode in place of TN (Twisted Nematic) mode is being attempted. The VA mode is suitable for display apparatus (or device) such as a flat panel display because of no optical leakage and excellent black. Moreover, the OCB mode is advantageously applicable for a flat panel display, particularly, a flat television, since the response speed is fast. However, in the liquid crystal drive system, use of the VA mode or the OCB mode in place of the TN mode has caused new problems. That is, in the VA mode or the OCB mode, birefringence is utilized for polarization rotation and polarization control is completed by disposing a polarization-controlling element on the light path. Therefore, there are some problems in use of a film having a large retardation (e.g., an in-plane retardation (Re)) such as current cellulose triacetate (TAC) film. In particular, with respect to a polarizer protective film, since such a protective film is disposed behind retardation film or a widen viewing angles (or viewing angle compensator) film (a WV film) in terms of light path in the side of the front surface, it is impossible to ensure optical compensation. As a result, there is a problem that the Re of the protective film is directly reflected in image quality of the display. Therefore, a protective film having a low retardation (particularly, a thickness retardation (Rth)) compared with the current TAC film is required.
Moreover, in a liquid crystal application for a large-screen television, there is a technical problem called “frame phenomenon”. That is, in the case where shock of temperature and humidity entail an action of a stress on a TAC film, phase difference is increased by change of retardation and accordingly optical leakage is generated. In a television which is a home electric appliance, it is also important to improve durability. Therefore, it is necessary to inhibit change of retardation even in the case where temperature and humidity are changed. To that end, of course it is advantageous to use a protective film having a low retardation value.
[Patent Document 1] JP-11-5851A (Claims, paragraph number [0005])
[Patent Document 2] JP-2002-212338A (Claims, paragraph number [0007])
[Patent Document 3] JP-2002-265501A (Claims)
[Patent Document 4] JP-2002-309009A (Claims)
[Patent Document 5] JP-2003-105129A (Claims)
[Patent Document 6] JP-2002-338601A (Claims, Examples)